1. Field of the Invention
The invention relates generally to a method of and apparatus for coining tapers at opposite ends of sleeve members. In particular, the invention relates to a method of and an apparatus for coining tapers at opposite ends of guide members for guiding the valve stem of valves for internal combustion engines, especially for guides made of a powdered metal.
2. Description of the Prior Art
An apparatus for and a method of coining tapers or leads at opposite ends of guides for valve stems of valves are known.
A conventional apparatus used with a crank press includes upper and lower cooperating dies to coin tapers at opposite ends of a guide sleeve. The lower die has a tapered bore defining a coining surface for producing the lower taper. The upper die has a tapered inner surface at an end thereof. A rod support member extends through the bore of the lower die for supporting a guide sleeve blank thereon. The guide sleeve blanks are loaded on the support rod and extend into the bore of the lower die to an upper edge of the tapered coining surface. During the coining operation, the upper die presses the blank downward so that tapers at opposite ends of the blank are coined. Upon finishing the coining portion of the cycle, a lower punch ejects the completed guide sleeve. A drawback of this method is that the end faces of the guide sleeves, which are made of a powdered metal, are susceptible to cracking. In practice, the rejection rate of such guide sleeves obtained by this method is very high.